A power supply is a device or system that supplies electrical or other types of energy to an output load or group of loads. The term power supply can refer to a main power distribution system and other primary or secondary sources of energy. Power conversion refers to the conversion of one form of electrical power to another desired form and voltage, for example converting 115 or 230 volt alternating current (AC) supplied by a utility company to a regulated lower voltage direct current (DC) for electronic devices, referred to as AC-to-DC power conversion.
A switched-mode power supply, switching-mode power supply or SMPS, is a power supply that incorporates a switching regulator. While a linear regulator uses a transistor biased in its active region to specify an output voltage, an SMPS actively switches a transistor between full saturation and full cutoff at a high rate. The resulting rectangular waveform is then passed through a low-pass filter, typically an inductor and capacitor (LC) circuit, to achieve an approximated output voltage.
Conventional series-regulated linear power supplies maintain a constant voltage by varying their resistance to cope with input voltage changes or load current demand changes. The linear regulator tends to be inefficient. The switch mode power supply, however, uses a high frequency switch, the transistor, with varying duty cycle to maintain the output voltage. The output voltage variations caused by the switching are filtered out by the LC filter.
Linear power supplies and SMPSs can both be used to step-down a supply voltage. However, unlike a linear power supply, an SMPS can also provide a step-up function and an inverted output function. An SMPS converts an input voltage level to another level by storing the input energy temporarily and then releasing the energy to the output at a different voltage. The storage may be in either electromagnetic components, such as inductors and/or transformers, or electrostatic components, such as capacitors.
A variety of different DC-to-DC power converter configurations are currently in use, most of which are variations of a buck converter, a boost converter, and a buck-boost converter. Some versions of the buck converter include the push-pull converter, the forward converter, the half-bridge converter, and the full-bridge converter. A resonant power converter includes an LC tank circuit, which operates such that the current through the inductor and the voltage across the capacitor are substantially sinusoidal. A resonant power converter includes an LC tank circuit. The LC tank circuit comprises one or more inductors and one or more capacitors, and the LC tank circuit exhibits at least one resonant frequency. In some cases in which the LC tank comprises more than one inductor or more than one capacitor, the LC tank circuit may exhibit more than one resonant frequency. Whereas hard-switched or soft-switched power converters are controlled by adjusting the pulse-width-modulation duty cycle, a resonant power converter is controlled by shifting a parameter of the circuit to cause it to operate at a frequency closer to or further from a resonant frequency. In a resonant converter, the transistor switches are typically switched in a manner that causes little or no switching losses by timing the switching to occur when the voltage across the switch or the current through the switch is close to zero.
A configuration using a push-pull converter is similar to the half-bridge converter configuration except that the push-pull converter configuration center taps the primary transformer. A configuration using a full-bridge converter is similar to the half-bridge converter configuration except that the full-bridge converter includes two transistor switches coupled to each end of the transformer primary, as opposed to one end as in the half-bridge converter.
The power factor of an AC electric power system is defined as the ratio of the real power to the apparent power, and is a number between 0 and 1. Real power is the capacity of the circuit for performing work in a particular time. Apparent power is the product of the current and voltage of the circuit. Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power can be greater than the real power. Low-power-factor loads increase losses in a power distribution system and result in increased energy costs. Power factor correction (PFC) is a technique of counteracting the undesirable effects of electric loads that create a power factor that is less than 1. Power factor correction attempts to adjust the power factor to unity (1.00).
AC-to-DC converters above 65 W, as well as some specific applications below 65 W, require the converter to draw current from the AC line with a high power factor and low harmonic distortion. Most conventional methods to produce a power factor corrected power supply with multiple isolated low voltage DC outputs include multiple converter stages and have significant problems with cross-regulation of the output voltages. The term “isolation” refers to isolating the input voltage from the output voltage. In particular, isolating means there is no direct conductive path between the power supply's input source and its output terminals or load. Isolation is achieved using a power transformer in series with the power flow from input to output. Isolation can be applied to the power converter as a whole, or to individual components within the power converter where the voltage input to the component is isolated from the voltage output from the component. Cross-regulation refers to regulating one element in the converter, such as one of several DC output voltages, while simultaneously regulating another element in the circuit, such as another of several DC output voltages. Furthermore, conventional methods require the isolation transformer to step down a high-voltage bus to low voltage outputs, a practice which requires a large turns-ratio and leads to transformer designs that are not optimal for electro-magnetic interference (EMI) or cross-regulation. These designs also typically include complex control to obtain input current waveforms with high power factor and low total-harmonic-distortion.
Conventional technologies typically fall into one of two categories for mid-power power-factor-corrected isolated converters. The first category uses a boost converter (step-up converter) to produce a high voltage bus, which is then cascaded with an isolated buck-type converter (step-down converter) to step the high voltage bus down to an isolated low voltage output. This technique is relatively expensive and not extremely efficient. The second category uses three cascaded converters with at least one of them, usually the last stage, being an isolated resonant converter. The three cascaded converters are also somewhat inefficient due to the fact that there are three stages. Furthermore, the final resonant stage presents great difficulty in achieving good cross-regulation of the output voltages.
A first conventional high power-factor isolated converter from the first category described above includes a boost converter to produce the high power factor input. Boost converter power factor correction circuits are limited in configurations. Voltage-source boost converters cannot be configured to provide an isolated output so another converter stage is included to provide isolation. Furthermore, boost converters are limited in their ability to be configured for soft-switching and resonant switching techniques, so these boost converters may produce large amounts of EMI, high losses (if operating at high frequency), and they often include expensive boost diodes to avoid problems with large reverse recovery losses in their diodes. Soft-switching, which can be accomplished through zero-voltage switching or zero-current switching, uses circuit resonance to ensure that power transistors switch at or near a zero-voltage level or zero-current level. This reduces the stress of the transistor component and also reduces the high frequency energy that would otherwise be radiated as noise. Hard-switching is the simultaneous presence of voltage across the transistor and current through the transistor when the transistor turns on and when the transistor turns off. This condition results in power dissipation within the device.
FIG. 1 illustrates a block diagram of a first conventional power factor corrected isolated converter. An EMI filter 12 is typically coupled between an AC input source 10 and the rest of the converter to prevent noise from coupling back to the AC source. The EMI filter 12 is coupled to a full-wave diode rectifier 14 configured to provide a rectified sinusoidal input voltage to the rest of the converter. A non-isolated boost converter 16 draws a nearly sinusoidal current from the AC input source 10 and charges a high voltage bulk capacitor to typically 250V to 400V, thereby generating a high-voltage bus. An isolated buck-type converter 18 and an isolation transformer 20 with multiple windings steps the high voltage bus down to one or more isolated low voltage outputs.
The boost converter 16 in FIG. 1 is typically hard-switched. Furthermore, to overcome high switching losses in the boost converter diode, the boost converter 16 typically either uses a silicon carbide diode, which is relatively expensive, or additional parts are added to enable soft-switched transitions, which is also expensive, or the boost converter uses critical or discontinuous conduction mode, which is applicable primarily to low power levels due to the extremely high ripple currents generated at the input of the converter.
The isolated buck-type converter 18, such as a full-bridge converter, uses a multiple-output winding transformer 20 to generate several levels of output voltage. Each of the secondary output voltages is rectified and filtered by rectifiers 22 to generate appropriate DC output voltages. In some cases, the DC output voltages are stacked, e.g. placed in series, to reduce effects of cross-regulation due to changes in the relative output loads. The stack is sometimes positioned after the rectifier, and sometimes before the rectifier, e.g. placing the transformer windings in series. For high-efficiency low-voltage outputs, rectification of the secondary output voltages is typically accomplished with metal-oxide-semiconductor field-effect transistors (MOSFETs) rather than diodes.
To reduce switching noise and increase efficiency, a second conventional high power-factor isolated converter from the second category described above uses a resonant output stage to provide isolation. However, due to the lack of voltage control of these resonant stages, a third cascaded stage is added to remove 100/120 Hz ripple from the high voltage bulk capacitor and to provide voltage control during transients. These resonant output stages also present substantial challenges to obtain cross-regulation of multiple outputs. In addition, the use of three cascaded stages reduces overall converter efficiency.
FIG. 2 illustrates a block diagram of a second conventional power factor corrected isolated converter. The second conventional power converter uses the same hard-switched boost converter as in the first conventional power converter of FIG. 1. In order to increase the efficiency of the step-down isolation stage, the second conventional power converter uses an isolated resonant converter 28 operating exactly at resonance. The resonant converter 28, when operating exactly at resonance, performs similarly as a DC transformer and does not control the ratio of input to output voltage. To remove effects of line-frequency ripple and load transient response from the boost converter stage, an intermediate third stage 24 is coupled between the boost converter 16 and the resonant converter 28. This intermediate stage is typically a non-isolating boost-type converter or a non-isolating buck-type converter.
The second conventional power converter shown in FIG. 2 has a number of problems. The use of three cascaded stages limits efficiency. The use of a resonant converter at the output stage puts significant current stress on the output rectifiers and substantially increases dissipation in the output rectifiers if MOSFETs are used to provide the rectification. The number of turns in each winding of the output transformer is often severely limited by the ratio of output voltages, and there is typically a high step-down ratio in the isolation transformer, which complicates design for low EMI and low proximity effect in the windings. The transformer also often includes a large air gap, which increases losses.
In a conventional application, a resonant converter is used for a halogen lighting application. It is common for many halogen lighting applications to use a half-bridge or push-pull resonant topology in which the output of the transformer is directly connected to the light-bulb, which is the resistive load. The resulting waveform on the input is sinusoidal. As such, the lighting circuit accomplishes power factor correction. However, such lighting circuits are designed to operate with a substantially single input voltage. The input voltage is “substantially” singular in that the lighting circuit is designed to operate over a relatively small range, for example 110V to 125V or 220V to 240V. Any change in the input voltage is automatically transferred to the load, so that during a brownout for example, the light bulb dims, and during a line swell, the light bulb brightens. Such resonant lighting circuits are not configured to enable universal input voltage operation. Furthermore, resonant lighting circuits have an AC output. The AC output drives the lamp directly with AC voltage.